Note: Descriptions are shown in the official language in which they were submitted.
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TECHNICAL FIELD -
The present invention relates to a method for reducing in a
digital time-multiplex mobile telephony system the risk of poor
reception when transmitting from a fixed station to a mobile
station, as a result of reflection of transmitted signals by
different objects, wherein each information frame to be transmit-
ted is divided into a plurality of information parts determined by
the number of interleaving levels.
BACKGROUND ART
In the case of radio communication between two stations of which
at least one is moveable, so-called Rayleigh fading will often
occur. This is due to reflection of the radio signals against
different objects, such that several signals which have travelled
along different paths will mutually coast or counteract one
another at different points. Consequently, the strengths of the
signals received will vary in accordance with the mutual positions
of the transmitter antenna and the receiver antenna, and may
2o temporarily drop to zero or to a value in the vicinity of zero.
Reflection of the signals also causes the direction of polariza-
tion of the transmitted radio wave to change in accordance with
the form of the reflective abject. The rate at which signal
strength varies is proportional to the re7.ative speed between the
stations. On those occasions when the relative speed is zero, the
connection may be broken because the input signal of the receiver
in one station is excessively low. For example, this can occur in
a mobile telephony system when a connection is established from a
vehicle which remains stationary in a geographic location where
the received signal strength is very low, or when the vehicle
moves slowly in such a location. Such locations are called zero
points.
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It is known that poor reception due to the occurrence of zero
points can be avoided with the aid of so-called space diversity.
In this case, a station, for example a base station in a mobile
telephony system, is equipped with two receiver antennas which
are spaced appropriately apart. A so-called diversity effect is
obtained in the fixed station with the aid of a suitable
addition function of two received signals. This is based on the
assumption that the probability of both antennas being located
simultaneously in a zero point is small.
In time multiplex mobile telephony systems with digital
modulation, the information from each subscriber is divided into
information sections, called information frames. These frames
are normally called speech frames in speech transmission. In
the case of GSM-type systems (Groupe Speciale Mobile), the
information in each such frame is divided into a specific number
of signal sequences (bursts), which together with corresponding
signal sequences from other subscribers form time slots in a
plurality of mutually sequential TDMA (Time Division Multiple
Access)-frames. Thus, each TDMA-frame includes information
parts, i.e. signal sequences, from several subscribers. The
number of signal sequences into which the information contained
in each information frame is divided is determined by the number
of so-called interleaving levels, the so-called interleaving
depth in the channel code. In the GSM-system, the interleaving
depth is equal to eight, i.e. the content of each information
frame is divided into eight signal sequences.
It is known to apply frequency-jump transmission in GSM-type
systems. In this case, different TDMA-frames are transmitted on
different frequencies. This method is based on the fact that
the positions of the zero points are frequency-dependent and
that the probability of a geographic location being a zero point
at more than one frequency is small when the frequency jump is
sufficiently large. The method is suitable for GSM-type systems
with deep interleaving.
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In radio systems having small cell radii and therewith
relatively short transmission distances, for example PCN-type
systems (Personal Communication Network), a frequency jump does
not provide any marked improvement of the reception at zero
points. This is due to small time dispersion, i.e. there is a
small time difference between signals that are received
subsequent to having travelled through different paths to the
receiver. Frequency jumps of reasonable magnitudes do not
change the geographic positions of the zero points to any
appreciable extent in such systems.
DISCLOSURE OF THE INVENTION
The object of the present invention is to provide a method of
the kind defined in the introduction which will avoid the
problems caused by poor reception that results from the
occurrence of so-called zero points, both in stations equipped
with only one antenna and also in systems having relatively
small cell radii and therewith relatively small time dispersion.
This object is achieved by transmitting from the fixed station
with a polarization direction which is changed stepwise between
different parts of the information transmitted. As a result,
the fading pattern around the receiver antenna of the mobile
station will be changed in time and any occurrent zero points
will move in the geography, even in systems having small time
dispersion, for example in city environments. The polarization
direction is changed for each new TDMA-frame in a GSM-system.
It is therefore improbable that more than one signal sequence
will be lost for a given subscriber.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail with
reference to the accompanying drawing, which illustrates an
exemplifying embodiment of an arrangement included in the fixed
station for the purpose of carrying out the inventive method.
BEST MODE OF CARRYING OUT THE INVENTION
The illustrated arrangement uses two antennas both for receiving
and transmitting. When receiving in the fixed station, diversity
reception is applied, although this is not a necessary feature of
the present invention. The antennas are referenced 18 and 19 and,
when receiving, signals received by the receivers are conducted to
a receiving device ~.1, via two duplex filters 16 and 17. Glen
transmitting, signals are generated in a transmission device 10,
and are applied to two branches. The signals in the two branches
are each conducted to a respective antenna 18, 19 via a respective
amplitude modulator 12, 13, a respective power amplifier 14, 15,
and a respective duplex filter 16, 17.
The antennas are oriented so as to have different polarization
directions therebetween. For example, t:he polarization directions
may differ by 90 degrees in relation to one another.
The amplitude modulators are controlled by signals from a control
means 20 and modulate the signals to power amplifiers sa that the
powers delivered to the two antennas are changed in relation to
each other. This results in a change in the polarization direction
of the resultant output signal. In GSM-type radio communication
systems with short Signal sequences, the amplitude should not be
changed during an ongoing signal sequence. This is because such
systems do not include an equalizer which can adapt to such
c~aanges. xn the case of the GSM-system, the polarization angle of
the resultant signal is therefore changed after each TDMA-frame,
by changing the amplitudes of the signals to the two antennas
stepwise after each TDMA-frame. The changes are preferably made so
that the polarization angle is changed 18o/N degrees for each new
TDMA-frame, where N is the interleaving depth in the channel code.
This means that Pt is equal to the number of signal sequences into
which each information frame from a subscriber is divided. If the
resultant polarization direction from the antennas is changed in
this way, it is probable that at most only one signal sequence will
be lost, even though the mobile station shauld be situated in a
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zero point for a given polarization direction, since the zero
point will move for each new signal. sequence.
It can be mentioned by way of. example that in a GSM-system in which
the interleaving depth is equal to eight, each information frame
is divided into eight parts which are distributed on eight signal
sequences. If each signal sequence has its own polarization angle,
it is improbable that two such sequences will give rise to a zero
point on the receiver. Transmission is therefore effected at eight
different polarization angles. These angles may, for instance, be
30 0, 22.5, 45, 67.5, ~0 degrees, etc. A single lost part of one
information frame can be reconstructed in the receiver, with the
aid of remaining signal sequences and a so-called error correcting
code.
The desired values of changes of the polarization angles between
two TDMA-frames are obtained when the control means 20 delivers to
the amplitude modulators 12 and 13 control signals which are
proportional respectively to the sinus and cosinus of an angle
which for each new TDMA-frame is changed to an extent which equals
the desired change of the polarization angle.
Time multiplex mobile telephony systems are also found in which
the signal sequences are relatively long in comparison with the
signal sequences of the GSM-system. For example, systems are found
in North Aznerica in which the interleaving depth is equal to two
between the signal sequences, but where interleaving also occurs
within each signal sequence. For example, each information frame
is divided into two parts, whereafter the second part of one frame
and the first part of the next frame are transmitted in the same
signal sequence. When the interleaving depth is equal to four, the
contents of said second arid first parts are divided into two parts
prior to transmission, whereafter these combined four parts are
spread appropriately within the signal sequence. For instance, if
half the signal sequence is lost, only a quarter of the two
original information frames will be lost.
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Systems of this kind use equalizers which adapt to changes in the
channel during a signal sequence. This change may be due, for
instance, to movement of a mobile station. The polarization
direction can then also be rotated during the signal sequences
instead of solely between different TDMA-frames. According to the
invention, each of the aforesaid four parts are transmitted with
a respective polarization direction. zn this case, four different
polarization directions are needed in order for all parts of a
signal sequence to be transmitted with different polarization
ZO directions. Generally, the number of different polarization
directions shall be equal to the number of interleaving levels.
This enables information parts to be transmitted with different
polarization directions from one and the same information frame.
It will be understood that the illustrated arrangement can be
modified in several ways. For example, separate antennas can be
used for transmitting and receiving purposes. It will be under-
stood from the aforegoing that it is not absolutely necessary to
use two antennas for receiving signals in the fixed station) The
fixed station may also be equipped with more than two antennas for
transmission purposes, wherein all antennas have different
polarization directions. Each of the antennas is supplied with
signals which have mutually different amplitudes.
